Production of dinitrotoluene mainly involves mixed acid nitration processes. Uses of nitrate as nitrating agent, such as sulfuric acid as a catalyst to react with aromatic nitro-aromatics, are a very important chemical process. Its main products include nitrobenzene, nitrotoluene, trinitrotoluene, nitro chlorobenzene and the like. The mixed acid nitration process may produce wastewater, which is highly colorful and has poor biodegradability. The main components of the wastewater include nitrotoluenes, toluene amines, acid class and multi-nitrophenols.
Treatment of mixed acid nitration wastewater is a hot spot in the field of environmental engineering. Developed countries generally use pretreatment involving stripping+thermal decomposition+ammonia distillation, which is expensive and difficult to expand. Some countries developed chemical treatment including a promising method: a wet oxidation method. Because nitrobenzene and nitrophenol are relatively stable under normal conditions and are not easy to break down, wet oxidation takes place at higher temperatures and under pressure. For example, a reaction temperature is generally 325 to 375° C., pressure of 2.20×107 to 3.45×107 Pa, the reaction time is 5 min. In such conditions, CO2, H2O and other simple small molecule compounds are generated out of oxidation of organic matters. According to a German patent (U.S. Pat. No. 5,356,539), nitro chlorobenzene or nitrobenzene waste water is heated to 100 to 300° C., under a pressure of 2×105 to 1×107 Pa with a catalyst, such as CuO, Al2O3 or magnesium or Cu, Cr, Zn oxide Al2O3, such that oxidative decomposition of organic matters takes place and more than 90% nitrobenzene and nitro chlorobenzene are degraded. However, there are problems with wet oxidation processes. These problems include high initial investment, high technical requirements (e.g., reactor sealed and heat-resistant materials), and requirement of additional investment for catalysts, which results in secondary pollution.
Methods for treating nitrification wastewater including chemical and adsorption techniques have been used in China. For example, some companies adopt an adsorption process in which adsorbents are mainly activated carbons and resins. The presence of activated carbons as an adsorbent for adsorption causes difficulties to recycle the adsorbent and to generates a lot of waste. This waste contains a large number of nitrobenzene substances as well as hazardous solid waste and incurs high processing costs. It has been reported that resin adsorption methods (e.g., methods related to ultra-crosslinked resins) have been used to treat nitrobenzene and nitro chlorobenzene waste. These methods are simple; however, the costs are very high and desorption solution needs to be processed.
Chinese patent application number: 200810121721 adopts acidification+iron+carbon reduction condensation+flocculation and settling pretreatment methods. Its process requires adding a lot of iron, carbon, and promote polymerization inhibitors (formaldehyde), flocculants (polyacrylamide, chloride, ferric chloride, ferric sulfate and ferric chloride polymerization), and produces a large number of iron-containing sludge, which are difficult to dispose. Chinese patent application No: 200910031041 adopt carbon reduction of iron oxide+Fenton methods+coagulating sedimentation process to treat mixed acid nitration wastewater. The method added calcium hydroxide in coagulation stations, resulting in a large amount of iron and calcium sewage sludge mud. This makes sludge became new contaminants, which are treated according to standards for hazardous solid waste. In these instances, the costs are high, and there are security risks associated with this method.
Techniques for treating electrochemical water, emissions, soil pollution have developed rapidly since the 1980s. Electrochemical methods and other methods have good compatibility and the combination thereof achieves the best treatment effect. Electrochemical treatment can be used to treat pollutants in sewage, waste gas and heavy metal ions, and its principles and methods include electrochemical oxidation, electrochemical reduction, photoelectrochemical oxidation, flotation and electrical power adsorption/electrical agglomeration. The electrochemical reduction generally occurs at a cathode, and the electrochemical oxidation can occur at an anode or cathode. The oxidation mechanism of the cathode is reduction of O2 to H2O2, thereby generating .OH oxidation of organic matters. Anode and cathode areas are generally separated by a membrane. Main forms of membranes include a salt bridge, an ion exchange membrane system and glass filter plate, etc. Ion-exchange membrane is divided into an anion exchange membrane and a cation exchange membrane. But electrochemically treated wastewater nitrotoluene is still in a conceptual stage. For example, Germany's BASF was the first company in Europe to try such techniques. The company filed patent application No: 201180025180.3 that described methods for removing nitroaromatic compounds. The method includes an aqueous composition including at least one aromatic nitro compound introduced into the electrolytic cell anode compartment and electrolyzed at an anode current density 0.1-10 kA/m2 and a cell potential of 4-15V; the cell has at least one anode; an anode including an anode section containing at least one section made of platinum or a carrier material and a coating formed; wherein the carrier material comprises at least one member selected from niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) metal, and the coating of boron-doped diamond. This is a bold attempt, but there are problems such as high costs and poor efficiency. Therefore, under conventional techniques, electrochemical methods for treating nitrotoluene wastewater incur high costs and complex operations.